But in your very first line you should subtly tell the audience, so there cannot possibly be any mistake, what your subject is.
Why are those two men out there on the stage?
What is the reason for their attitude toward each ther?
If they are quarreling, why are they quarreling?
If they are laughing, why are they laughing?
But don't make the mistake of trying to tell too much. To do that, would be to make your introduction draggy. You must make the audience think the characters are bright—precisely as the introduction of the monologue is designed to make the audience think the monologist is bright. Write your introduction in very short speeches. Show the attitude of the characters clearly and plainly, as the first speech of our two-act example shows the characters are quarreling:
STRAIGHT
Say, whenever we go out together you always got a kick coming.
What's the matter with you?
Then get into your subject-theme quickly after you have given the audience time to get acquainted and settled, with the memory of the preceding act dimmed in their minds by the giggle-points of your introduction.
The introduction of the two-act is designed to stamp the characters as real characters, to establish their relations to each other, to give the audience time to settle down to the new "turn," to make them think the performers are "bright" and to delay the first big laugh until the psychological moment has come to spring the initial big point of the subject theme, after the act has "got" the audience.